At airports, pilots orient themselves during takeoff and while on the movement area by means of airport visual aids that are set in the pavement along the taxiways and/or runways. These airport visual aids must operate extremely reliably and must reliably guide the pilots particularly in the event of poor visibility conditions, for example, during periods of darkness or fog. If individual lamps of the airport lighting system fail, this failure must be quickly detected and corrected in order to guarantee safety. Because visual inspection would be time-intensive and expensive due to the large number of lamps, a detector circuit is typically provided for detecting defective lamps. The individual lamps are typically powered by means of individual supply transformers, wherein the detector circuit is allocated to the supply transformer.
For many years, conventional lamps, especially halogen lamps, have been the most cost-effective and most reliable light source for airport lighting systems. Detector circuits that have been tested and proven for many years are available on the market for such halogen lamps. For example, U.S. Pat. No. 3,061,828 describes such a circuit. The circuit makes use of the fact that when there is a defect in a halogen lamp, the transformer goes into a saturation state. The saturation state changes the amplitude and phase of the voltage. This change can be detected by the detector circuit and thus a defect or failure of the light source can be reliably determined.
The detector circuits known on the market are designed for halogen lamps. They cannot be used without additional components in modern visual aids with LED light sources, like those described, for example, in U.S. Pat. No. 7,654,720, because of their different behavior in the event of a defect. In light of this background, the published international patent application PCT/EP2012/059154 by this applicant discloses an adapter circuit by means of which, in the event of a defect, the behavior of an LED visual aid should approximate the known characteristics of a halogen lamp. Although this adapter circuit has been tested and proven in principle, errors in the fault detection can occur especially due to a residual current that also flows through the adapter circuit in the event of a fault. Moreover, when switched on after a fault state, this adapter circuit exhibits a delayed response. Thus, after being put in operation again, there will be a delay in detecting a defect that has not been corrected.
Consequently, the problem of the present invention is to disclose a modified adapter circuit that is designed for an LED airport visual aid, reliably detects a defect, and exhibits improved response when it is put in operation again.